Consider using stub instead of spy

Since routes usually trigger dom changes such as

```javascript
var Router = Backbone.Router.extend({
  routes: {
    '': 'index'
  },

  index: function() {
    var layout = new Backbone.View();
    $('body').empty().append(layout.render());
  },
  ...
});
```
Using `chai-backbone` route assertions will break the testrunner page by actually rendering the view(s) into the DOM. 

I changed the `sinon.spy` to `sinon.stub` to exclude actual method execution.